text:
Can quadcopter-like UAVs ever get significantly quieter? Any candidate technology?
note: this question is old (2016) but so far remains unanswered. Perhaps innovations have improved on the noise even since this was first posted.

The light weight quadcopter and hexacopter UAVs that can cary say 2kg to 20kg payloads for "delivery" produce enough noise that a steady stream of pizza and book deliveries might not be so welcome in many populated areas.
Are there any candidate technologies that can significantly reduce the noise heard by people on the ground? One possibility is of course to fly higher, but that may not always be possible or desirable, and I don't think they'll be deploying pizzas and books with little parachutes from 500 or 1000 foot heights.
Better propeller design? Cleverly ducted/muffled thrust? Anything?
I'm sure possible techniques will interfere with thrust, so anything that has an impact of less than 50% would count here. Batteries will continue to get better over time, but I can't imagine much more innovation can be done with propellers.
I'm not looking for guesses, but it doesn't have to be proven on this scale either. My feeling is that there really isn't anything besides distance that can help very much.

I'm not understanding the parachute point. Couldn't you fly in high, then drop vertically under reduced power? You'd need a quick burst of power just above the ground to decelerate, but the subsequent takeoff would be unloaded and hopefully less noisy.

@MSalters that's a good idea. If it's really quick, then the guy next door won't have time to run outside and shoot at it. In the mean time I'd like to understand if there is "anything besides distance that can help" reduce ground noise.

A quiet fan is a large fan.
(This should be understood in relation to either mechanical power or lift. Of course, a multi kilowatt industrial turbine is way noisier than the fan cooling your computer.)
A single large and slower propeller (or two coaxial) instead of 4 or 6 small ones will do wonders, provided one can get good enough automatic pitch control. A better energy efficiency will be a bonus.
We just invented the helicopter.

Edit:
There is a fundamental difference in how rotors of quadcopters and helicopters work.
The quadcopter has fixed blades and variable speed. To vary the speed of an electric motor is easy, the whole thing is simple. But: we need at least 3 rotors (and things get easy with at least 4) in order to control the vector of the lift. Rotors are small, they rotate at high speed. Results:

blades have some compromise form, they are not optimal at any speed
blades have some compromise angle of attack, again not optimal in any condition
blades are small for the load they lift (no place for bigger ones).

These three things mean a constant partial stall for some part of the blade. A lot of energy is lost to sound and vortices.
On the other hand, on a heli we have:

single rotor with big blades, running at more or less constant speed, optimized for lift, drag and noise at that speed
the angle of attack (pitch) of the blades is precisely controlled at every revolution. That's how both the lift amount and the lift vector are controlled (keywords: group and cyclic control).

The reason why small UAV are not usually made as helicopters is the pitch control. The mechanism is complex and maintenance-heavy, its control algorithm is complex as well.
On the other hand, the technology constantly improves and we COULD get something workable in the near future.
On the good side, helicopters powered by internal combustion have the limitation of the engine speed not being able to change much and not being able to change quick. A variable pitch AND a variable speed of the electric motor may offer additional means of control and efficiency improvements.

In regard to comments that helicopters are noisy as well:
The lightest one is probably ~500kg, there are ones that are tens of tons. This is the main reason why they are noisier than a quadcopter.
I don't want to even imagine the sound of a 4-rotor 1000kg copter.

Comments are not for extended discussion; this conversation has been moved to chat.


text:
How does the University of Bristol's AI fixed-wing perched landing change the wings?
The BBC radio program Click's 2017-01-24 episode, starting at about 06:20, describes the use of AI to develop a perched landing capability by an autonomous fixed-wing craft. I can't understand exactly what is happening.
While the BBC's Gareth Mitchel suggests the winds "sweep up", BMT Defence Service's Antony Waldock says that they move or rotate forward, and goes on to say, roughly transcribed:

...as you are about to stall you rapidly pitch-up the aircraft, and that pitch-up gives you a little bit of extra lift, and that stops you (from) kind-of mushing into the ground, so you can 'pick it up', and land with minimal velocity at a particular location.

BBC's Bill Thompson then describes the technique as a change the wing shape allowing a controlled stall to lose velocity really quickly and manage the descent.
I'd like to understand if the wings "sweep up" or do they move forward, and is this movement considered a change in the wing's shape? Or are they otherwise changing shape?

Maybe this news article helps or This YouTube video.

@RonBeyer this video attempts to explain in detail (though still ends with a stall and fall). Would you be interested in summarizing as an answer? This isn't my area of expertise; I'm not likely to get it right if I try to answer myself. Thanks!

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Did the Phantom F-4 contain "polychron" or other synthetic materials as a replacement for rubber circa 1969?
update: In order to make it easier to answer this question, use of "synthetic materials as a replacement for rubber circa 1969" in any aircraft might be good enough.

Michael Crichton's 1969 science fiction thriller The Andromeda Strain and the 1971 film of the same name ficticiously describe/depict a Phantom F-4 where a biological agent recovered from a satellite and subsequently mutated degrades and destroys rubber-like materials in the aircraft including the pilot's oxygen mask, and eventually the pilot as well.
In this clip from the film an expert at the site of the crash says:

There is no actual rubber on the phantom F-4 general. It’s all a synthetic plastic compound called “polychron”. It has some of the characteristics of human skin.

The spelling is in the computer-generated closed captions. I can find an insecticide Profenofos which is also called polycron, but nothing else.
Crichton is famous for meticulous technical research (generally) for his books, though they are still of course fiction.
Because of that, I'm wondering if the plot device of "polychron" replacing rubber in the F-4 has some basis in fact.
Question: Did the Phantom F-4 contain "polychron" or other synthetic materials as a replacement for rubber circa 1969?

In the novel, no name for the polymer is given.  "It was all a synthetic plastic compound. Newly developed by Ancro; they're quite proud of it. It's a polymer that has some of the same characteristics as human tissue." P. 196 of the 1973 printing of the Dell paperback.

@OrganicMarble I have a hunch you didn't have to run down to the public library to find that reference ;-)

guilty as charged.

I haven't worked on the old fighter jets, but in all of our studies of construction and composites, i've never heard of something called polychron. Best case is someone around here who worked on the F-4 can give an answer :)

However, after WW2 is when the industries really started experimenting with materials, so i'm sure there would have been some sort of synthetic material replacing rubber in the aircraft around 1969

One of the most commonly used masks during the Vietnam War era was the MBU-5/P Oxygen Mask and was made out of "natural" rubber. Later masks incorporated "silicone rubber," so whether or not that constitutes a "synthetic material" is a matter of semantics.

Personally I'd think that any material that bends and stretches like rubber, but is not derived from latex from a plant, would qualify as "synthetic".  In my opinion silicone "rubber" certainly qualifies.  Were the silicone rubber masks in use in F-4s by 1969?

I couldn’t find a MILSPEC with an effective date in or around 1969. The first mention of silicone rubber I found was from the early 80s, but that isn’t as definitive as folks are wanting for a fictional situation.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How does a fuel density sensor work? (densitometer, cadensicon)
While some fuel volume sensors use ultrasonic measurements this answer to How is fuel mass measured in airliners? says:

The FCOM lists the sensors involved in this measurement:

The FQI system comprises : [...]

A set of capacitance probes in each tank to measure fuel level and temperature.
one densitometer (cadensicon) sensor in each wing inner tank permitting the calculation of the fuel quantity.
one Capacitance Index Compensator (CIC) in each inner tank giving the dielectric constant of the fuel in case of cadensicon failure.

(Airbus A320 FCOM - Fuel Description - Fuel Quantity Indication and Level Sensing)
Using the densitometer (or the backup CIC), the conversion from volume to mass is performed by the FQIC. The ECAM can then display fuel temperature and mass as calculated by the FQIC.
(Airbus Safety First - Fuel monitoring on A320 Family aircraft)

So in this case capacitive sensors (plus fixed tank shape) can determine the fuel volume, and a density measurement can convert that to a mass.
Question: How does a fuel density sensor work, being the "densitometer" or "cadensicon" mentioned in the block quote.

It reads a bit like you wouldn't need the densitometer in case of ultrasonic sensors, but you need them there as well.

I'm referring to your introduction - "While some fuel sensors use ultrasonic measurements this answer to How is fuel mass measured in airliners? says: ..." But yeah, the rest is fine. And for the topic: you can calculate density with a differential pressure sensor and a known height between them, but I have no idea what they use in aviation.

@Arsenal Oh, yes I see what you mean now, sorry my bad. I've edited to differentiate. Thanks!

I don't know what exact kind they are using in the A320 (from which the quote in the question is). @mins found a forum post about an Airbus A300-600 cadensicon:

The cadensicon measures fuel dielectric constant and density.

Unfortunately, that source does not describe how the density is measured.
However, I found a densitometer that is used for aviation fuel density measurements. It measures the change in resonance frequency of a  spring, which is driven by a piezo. That frequency change depends on the fluid density, which allows calculating the density from the measured frequency and known calibration coefficients:

The densitometer uses a torsional mode natural
frequency to determine density in real time, providing an
accurate and reliable indication of fuel density over the
temperature ranges typically encountered in aircraft fuel tank
applications. It is designed to operate in all aircraft fuels
encountered worldwide.
One end of a spring is driven torsionally to resonance using an oscillating electrical
signal by a piezoelectric crystal, the other end of the spring is allowed to freely
vibrate at its natural frequency. This end is “coupled” with the fluid media and
natural frequency changes in response to the fluid density are measured on a
second drive piezoelectric crystal.
The primary output is an AC voltage having a frequency that varies with density.
Density is calculated from the output frequency via the equation:
$ \rho = a + b / \text{frequency}^2 $, where a and b are constants determined during calibration.
The standard method of acquiring the frequency in a microprocessor based system
is with a timer/counter chip having a two-second gate time insuring high resolution.

(AMETEK Aerospace - Fluid Densitometer)
For more details, see the patent US4275585A.

Short answer
The two cadensicons used on A320 are float type, density is determined by measuring Archimedes force (buoyancy). A float is maintained by springs and the force on the springs is measured.

The force "is equal to the weight of the fluid that the float displaces".

This weight depends on different elements: volume, gravity field and density.

Volume is constant, gravity field is assumed constant, leaving only a dependency on density.

Details
A cadensicon, or capacitive density condensator,  is a densitometer with a measure of fuel permittivity (dielectric constant), to calibrate the capacitive probes.
On the A320, there is a cadensicon for each tank (2 or 3 if there is a center tank), all float type. View of the A320 cadensicon:

A320 cadensicon. Left: Boeing site, right: Airbus A320 documentation
From Airbus technical training manual for A320 fuel system:

Each cadensicon houses 2 sensors covered by fuel. A fixed capacitor covered by fuel is the primary source for dielectric constant of the fuel in each tank and a densitometer float measures fuel density. Both parameters are used for calculations of the fuel quantity.
As density of the fuel grade in use changes with temperature, a fuel thermistor makes an allowance for temperature variations.

A partially immersed probe can be seen as the sum of two variables capacitors with different dielectrics (air and fuel) and which total capacity is a function of fuel level. Each tank has a probe fitted with a capacitance index compensator, which measures fuel permittivity at the bottom of the tank.

A320 fuel level probes, from Airbus A320 documentation
The location of the probes is chosen to ensure at least one probe cuts the fuel level in any aircraft attitude.
The FQI has two channels, one working to compute the mass, the other to monitor the first one. Mass is computed from the levels sensed by capacitive probes and:

Permitivity from cadensicon
Permitivity from capacitance index compensators
Density from cadensicon
Fuel temperature
Pitch and roll angles.

The conversion of levels to volume takes into account the attitude determined using the probes and a table in the computer memory containing the geometry of the tanks.


text:
How does ATC know if you have been assigned a squawk or not?
Departing VFR from class D airspace, either you can request flight following after you leave the airspace or you may ask the airport ground/tower and they assign you a squawk code and setup the flight following for you.
For example, on a short flight from KCNO to KSNA, I usually get my squawk code from tower and after departure I switch to SOCAL and let them know I departed KCNO and usually follows by "ident".
How does ATC know if you already have a squawk code assigned to you or not?
How does initial communication with SOCAL differ from when you've been assigned squawk on ground before take off from when you want to request one?

When you hit "ident" you will start blinking on the screen. The number will be assigned in "the system" which ATC uses to track flights, just like if it were IFR. Sometimes (or a lot of times) when getting handed off I'll get a revised squawk code instead of them using the one that the local tower assigned me.

also, the Class D tower doesn't assign you the squawk code. The tower contacts TRACON ("departure"), which assigns the code. The tower just relays the message.

My tower almost always declines to help with a Squawk or Flight Following.  They tell me to contact Center.  I don't even bother asking the tower.

@J.Hougaard's answer is excellent (as usual), but it doesn't address the last part of your question:

How does initial communication with SOCAL differ from when
  you've been assigned squawk on ground before take off from
  when you want to request one?

If you have a discrete code, then you are "handed off" from one facility or sector to another. This means that the sending controller does something on his console to transfer responsibility for your flight, which prompts the receiving controller (who may be at the next scope or hundreds of miles away) to accept the transfer. After the transfer is accepted, the sender tells you by radio to change frequencies and call your new controller who, having just accepted you, will be expecting your call. This call, commonly referred to as "checking in", needs only contain your tail number (or callsign/flight number, if applicable) and altitude; the main purpose is to let the receiving controller know you're now on their frequency.
If you don't have a discrete code, then you're just an anonymous blip on the controllers' screens, so you are not just checking in. You will need to request flight following and give all the information (type, destination, cruise altitude, etc.) that you would have given on the ground, and they will give you back a code and start looking for you on their scope.
Either way, a controller may ask you to ident because it makes your target stand out on their scope, which saves them time. If you gave an approximate position when you called, this is less likely since they'll know where to be looking. Don't ident unless they ask, though, since its not always needed and may distract them when they're looking for someone else.

Are you an US ATCO? Is this really how it works over there? I am ATCO in Germany and what you write is not quite the case here.

We have the same thing called Flight Information Service (FIS) by dedicated personell on dedicated frequencies - which makes sense because I think the VFR traffic in Central Europe is probably much more crowded than in the US. FIS stations here have 40-50 aircraft on frequency sometimes. But that‘s not what I meant: You wrote that the sending Controller has to do sth to initiate transfer and only after transfer was accepted he may send the aircraft to the next frequency. That’s nothing we are used to over here. We just send the aircraft to the next freq., no coordination necessary.

OK, I see. So your systems are not trajectory based? European systems are really modern today. In Germany for example only aircraft who are planned for your sector are blue because the system calculates a 4D-trajectory and knows exactly when an aircraft enters which sectors. Once the previous controller sends the acft it will change into send state on my screen. I can see that it is send, but I won‘t assume the label until the aircraft called on my freq. All aircraft on my frequency are black and the others outside my sector are white so you can see them but they don’t distract you.

Oh this sounds like it‘s limiting your ability to grow with the traffic increase as well? Or it requires hard work by the ATCOS to compensate? In upper airspace in the meantime we work roughly 60 aircraft per hour in a sector extending from FL320-FL360 with a 70nm by 70nm shape.

Let us continue this discussion in chat.

Agree with @pcfreakxx - this answer seems to be very US-specific. Generally speaking, uncontrolled VFR flights are not coordinated.

The OP asked about US flight following between US airports and referenced US ATC facilities and procedures. Do I really need a disclaimer that the answer is for the US?

Let's remember what a transponder code is used for in the first place.
When transmitting a non-discrete code, such as 1200, ATC has no way1 of telling you apart from other aircraft transmitting the same code. On the ATC radar, you will simply show up as something like "1200/A030" where 1200 is the code transmitted and A030 (3000FT) is you mode C altitude.
To provide you with radar service, ATC has to be able to identify you on the radar, which means that they have to be able to distinctly associate your flight with a specific track on the radar. They do this using transponder codes2. An assigned (discrete) transponder code will be unique for a flight within a given area. Say you are instructed to squawk 2174, you will now show up on the screen as "2174/A030", and because ATC knows that you, and no one else in the area, is squawking 2174, they know that they are you looking at you.
Your question is, how does ATC keep track of the assigned transponder codes? Today this is mostly an automated process. A central flight data processing system (or the US equivalent - sorry for my European terminology) contains a database of all active flightplans as well as their assigned transponder codes. When a discrete transponder code is received by the ATM system, it will automatically query this database, asking for any flightplan information associated with this specific transponder code. It will then show the received information on the radar screen. Essentially, the transponder code is a database key, allowing the system to look up information about your flight and show it to the controller.
Now you are no longer shown on the radar with just your transponder code and mode C altitude, because the system has a lot more information about you. You might be shown as something like "N12345/A030/C172" (callsign/altitude/aircraft type). The exact label varies greatly from system to system - the point here being that the system is now able to show specific information from your flightplan rather than just generic information submitted by your transponder.
So how does a controller know if you have been assigned a transponder code or not? If you have, you will be shown on their radar screen as a full label, with callsign etc. If not, your callsign is no where to be found on the radar, and they will have to guess that you are probably one of the (potentially many) 1200-tracks on the screen.
1: They can in fact identify you by other means, such as requesting your exact position and heading, but identification with the help of transponder is a lot simpler.
2: Normal 4-digit transponder codes are slowly being replaced by mode-S transponders, which transmit a unique 24-bit aircraft identifier at all times to the radar system. But that's probably best left for another question.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Do reduction drives increase tach time quicker than direct drive engines?
For example, the Rotax 912ULS engine has a 2.4:1 reduction. Does this mean that the Time Between Overhaul (TBO) will be reached 2.4x faster than in a direct drive engine or is tach time reduced by 2.4:1 as well?

Does the 2.4:1 reduction reduce to rotation speed of the drive shaft, or the progression of time?

Not sure what you mean by "Progression of time". Are you joking?

The relationship between tach time and RPM varies based on the engine. As mentioned in the linked answer, it is difficult to find this relationship based on engine specifications, but if you find a tachometer certified for the engine it will have the correct ratio built in.
If the 912ULS needed TBO based on tach times, the tachometer would likely count tach time at 1:1 somewhere below 5500 RPM, since that is the maximum continuous speed for this engine. However, Rotax actually uses Hobbs time to determine when an engine needs a TBO, and specifically notes that a "mechanical hour meter" may give different times. Therefore, the TBO of 2000 hours is 2000 hours of real-world time.
From the 912 Series Maintenance Manual: (p. 3)

All of the maintenance intervals, such as the 100 hr. inspection and the engine TBO, relate to the number of operating hours of the engine.
The operating hours are defined as follows in order to prevent misunderstandings and to ensure safety:

All time during which the engine is running is counted towards the total number of operating hours.

The time is counted irrespective of the load factor of the engine, such as idling or take-off power.

NOTES: A mechanical hour meter is directly coupled to the engine speed, the readings may deviate considerably from those given by electronic remitters (e.g. TCU, FlyDat). Maintenance and overhaul intervals are always dictated by the readings of the electronic hour meter.

I never realized the tach time varied based on the engine. I had always thought it was standardized to 2400 RPMs. Thanks!

@jaytre It's actually based on the tach installed - we discussed it a bit here. Interestingly as best I can determine the tach is an airframe component (so Piper could spec a tach with a 2300 RPM equivalence, but Cessna could spec one with a 2200 or 2400 equivalence on an otherwise identical engine).


text:
How should I refer to a transition through class B or class C airspace?
In Seattle, it is common to request a transition over KSEA by asking for either the "Bremerton Transition" or "Kent Transition" (depending on what direction you're travelling). I learned these names from my CFI & have used them with no problem, but I've noticed that on the Seattle TAC they are referenced as Seahawk/Mariner Transition, which refer to which runway numbers to overfly rather than direction (personally, I've never heard these referred to on the radio).
Obviously, "Bremerton" transition refers to transitioning to Bremerton, but I'm curious if this is some agreed-upon lingo or just convention. Could I just as easily ask for a "Port Orchard" transition?
If I were in an unfamiliar area, could I request a transition by referring to any location on the opposite side of the airspace? For example, if I wanted to transition PDX airspace could I request a "Hillsboro Transition"?

Sidenote: I personally love transitioning KSEA. If you haven't done it & are in the area, pick a busy day and request a transition. It's pretty impressive to see the large commercial flights landing right below you. On a non-busy evening, I've heard rumor you can even request a stop\go on 16R

I have added the [tag:faa-regulations] tag, as transitions through C/D class airspace in e.g. Germany works differently and the ATC having control over the airspace does not have detailed information about cities or landmarks on his radar scope. If you are interested, I can provide an EASA-based answer, but if you are only interested in FAA-procedures, I am unable to answer.

@SentryRaven - you should provide the EASA answer; I'd be curious to know what the situation is in the EU.

@SentryRaven - I was originally interested in FAA, but now that you mention it I'm also curious about how EASA differs. If you posted an EASA answer, you'd get my upvote...

You'll sometimes get radio phraseology different from what is published on charts either due to some holdover from now-superseded procedures (bad form, but it happens) or because of Letters-Of-Understanding that exist between the local Air Traffic Facility and certain operators or interest groups/associations, etc.
If you're going someplace new and want to use a published VFR Transition through class Bravo, the best way to go about it is to use the nomenclature on the charts in order to reduce confusion. Otherwise, it is also acceptable to simply request a VFR clearance through the airspace in a general direction of travel. Depending on your altitude, position and direction of travel, as well as traffic load and any airports you need to get around, ATC will vector you through or around the class Bravo.
The initial call follows the standard procedures outlined in the AIM and the Pilot/Controller Glossary. For example, starting out NE of Orlando class Bravo and asking for a transition to the SW, could be something like:
"Orlando Approach, Husky 123 Alpha Bravo Charlie, 35 northeast of the ORL VOR at 4500, request VFR transition to the Southwest through class Bravo."
If you're near a local landmark, you could use it as a reference to help the controller find your aircraft more easily:
"Orlando Approach, Husky 123 Alpha Bravo Charlie, 2 northeast of Lake Ashby at 4500, request VFR transition to the Southwest through class Bravo."
Generally speaking, if a VFR Transition or flyway is available, the preference is to have VFR traffic use it in order to lighten the controllers' workload, but if you don't have the charts, or would rather not do that for some reason or the other, you don't have to. Just follow the general procedures for transitioning through class Bravo and they'll do their best to accommodate you.


text:
Is it right to extend flaps only in the white arc?
If the cruise speed is >80kts on the downwind leg but I can only extend flaps in the white arc, am I right in saying I need to get to the white arc, even if I'm on - for example - 100kts?
For reference, note the 'white arc' on this airspeed indicator.

What airplane are you talking about, and how much flap are you adding?

I don't entirely understand your question. Are you asking whether extending flaps above the white arc is acceptable? Or something else? What has 100kts got to do with it?

"If  I can only extend flaps in the white arc, do I need to be in the white arc to extend flaps?"  Is that really your question?

Possible duplicate of https://aviation.stackexchange.com/q/5121/7147

Yes you must slow down to the white arc, or whatever your flap extension speed is for a given condition, regardless.  If you are 10kts above the white arc and drop flaps anyway, it's not going to make the airplane come apart, and if you did it once, slap yourself on the wrist and don't do it again.  It's putting stress on the flap attachments beyond what they are certified for, and eventually if it's done repetitively over a long period you might start seeing cracks in hinge arms, brackets etc.
On transport airplanes if you do that you get an overspeed warning and you are expected to declare the incident which will trigger a fairly straightforward visual inspection of the flaps.   On a small a/c, if nothing is bent, don't worry about it if it was just a few knots over, but don't keep doing it.  If it was WAY over, like 20 or 30 kt above the white arc, you should tell whomever owns or maintains the airplane about it so someone who knows what to look for can look it over.
In fact, as a matter of good airmanship, you should get into the habit of being well below the top of the white arc, within reason, before extending flaps (I slow down to not much above the flaps up approach speed before dropping flaps - it's a matter of being on top of how your aircraft flies, and staying ahead of it, mentally).  This is simply a matter of taking care of the machinery and not beating the crap out of it; it'll mean the difference between having to replace hinge bushings at 5000 hrs for the pilot who takes care of the equipment vs 2000 hrs for the pilot who treats his/her plane like a mule.

I suspect one also shouldn’t beat the crap out of their mule.

@KRyan it depends - is it a rented mule?  :D

Yep the mule can eventually get pissed and kick you...

It depends upon what the manufacturer recommends on the form of the AFM in cockpit placards.  Certain aircraft do allow extension of the flaps outside the white arc for certain flap settings.  For example the Cessna 182T is placarded for 10° flaps at or below 140 KIAS, 20° flaps at or below 120 KIAS and 30° flaps at or below Vfe of 100 KIAS, despite the fact that the airspeed indicator is placarded for a flap operating range of 41-100 KIAS.  Check in your Airplane Flight Manual under the section for design limitations.

It is quite likely that the OP's aircraft allows some flap extension at 100 KIAS. But if you don't know how much, then you really have no choice but to stick to the white arc. But the flap extension requirements is definitely something the PIC of an aircraft should know.

@David Schwartz that’s all been determined during design and flight test and should be listed in the operating limitations section of the Airplane Flight Manual.


text:
How dangerous was the turbulence on this commercial flight?
EDIT: Having read these great answers I feel a little ashamed for the fear I felt on this recent flight. However, judging by my reaction to the experience and the reaction of those around me on the flight I imagine I am not the only one who felt this way. As others have mentioned here I will agree that passengers are pretty bad at judging turbulence. For those who come to this Stack Exchange site with similar concerns, I hope that you read these answers from those knowledgeable in the field to assuage your concerns.

I was recently on a flight that flew in to central Texas coming from Denver. There was bad turbulence on the flight in the last 45 minutes -- the worst I have ever experienced -- and I was curious about what else was going on behind the scenes from the perspective of the pilot and crew.
I do not know the airplane model. It has ~38 rows and 3 seats on each side of the aisle. I was in row 35. It was a standard commercial airplane that would fly across the United States.
We were first told that we were about to enter a turbulent area based on reports of other airliners in the region. We soon entered an area of lightning and dense clouds so thick that there was no visibility outside the windows.
The first drop came unexpectedly in what I can only describe as similar to a bad, bumpy landing on the freeway. The bags rattled pretty hard and loudly and I felt myself leave the seat. It was loud and jarring enough for most of the people in my area to scream out loud. Over the next 45 minutes, these drops happened continuously and randomly, up to 15 times.
Between these big drops the plane felt like it was swaying from the left and right. There were also many tiny dips between the bigger dips. It was extremely tense for everyone on the plane and generally it felt very uneven below us so there was a constant anticipation of another drop.
After the landing gear went down, it was clear that we were ascending again. The pilot explained that there was a "spacing" issue on the ground so he was going to fly around and try to land again. The guy next to me said (jokingly?) that this meant the pilot was too scared to land.
The cabin crew looked pretty terrified, and they were chatting to each other throughout the whole ordeal. People were sobbing and weeping all around me and they would yell every time a drop happened.
Obviously, you have little information to go off of but I was curious, from what I have provided, about the following questions:

Many people on board were fairly traumatized by the experience. Was there any real danger from turbulence this bad? Is this something a pilot might encounter routinely that they would be equipped to handle or does this seem unusually bad?
What was happening behind the scenes? How does a pilot handle a situation like this? Was there other information about the turbulence that we were not being told?

Any other interesting, related information to give me insight into this experience would be appreciated.

If the pilot said the go-around was for spacing, he was probably telling the truth. It's a very common reason for go-arounds/missed approaches. If you want to be sure, though, if your flight was in the last 14 days, you can listen to the ATC recording for your destination airport at liveatc.net. I've done that before when I was curious about the reason for a go-around on a flight I was on. In that case, they were just coming in too fast and couldn't stabilize the approach quickly enough (because we were basically dive-bombing the airport after flying around a thunderstorm.)

If there was thunderstorm activity around the airport, it's likely that some takeoffs were delayed, planes are taxing slower, and approaching planes are re-routed around thunderstorm instead of their usual route. Give all this, it is not unusual to expect spacing problems.

Re: your EDIT. I don't think your fear is unfounded.

@rbp I don't think so either... but I always feel a little humbled when I learn the greater context of any situation. I learned a lot from these answers and so with that new perspective I can look back on what I felt before. I hope other passengers will read this and do the same.

I always throw up my hands and go WEEEEEEE! as if it's a roller coaster ride.  It's about as safe as one, so might as well enjoy the freefalls - a lot cheaper than a trip on the vomit comet.

oh boy, this is nothing. @rbp i think the fear IS unfounded, because OP was never in any danger (well, no more than usual on a plane). that's the definition of unfounded fear. OP's lack of knowledge led to the feeling of fear. happens to all of us.

wow -- i got that completely backwards. yes, his fear is unfounded.

@rbp: No, it's "lightning"; your edit was incorrect.

@LightnessRacesinOrbit thanks, fixed here and below

I'm surprised nobody asked for flight number and date. That would allow a quick look at historical weather as well as flight track to get an idea of what was going on.

Also, I'm surprised everyone missed the obligatory

Please don't be ashamed about the question, its a great question. I don't fly airliners, but I feel just the same as you do in turbulence, when I'm not driving myself. I've been out on some gusty days myself but flights on airliners are a different matter. You are sat in the back waiting for the next bump wondering which part of your stomach is going to fly the furthest. When driving, I've had my head hit the roof in turbulence (not all light aircraft have four point harnesses). You just get on with it and concentrate on the job in hand.

@AdamDavis At least as far as the major airlines are concerned, it's actually probably more safe than a roller coaster.

@reirab A quick search suggests that chance of death in airplane is 1 in 11 million, while chance of death on roller coaster is 1 in 24 million, so I was wrong - you're twice as likely to die flying than riding a roller coaster.  But then roller coasters don't even make it onto this chart while vehicle and airline deaths do, so I feel pretty safe in saying that flying is still more dangerous than rollercoasters, but it really depends on how you slice the statistics.

@AdamDavis Chance of death in an airplane depends very much on the type of airplane, who's flying it, whether you're measuring per flight or per mile, and how far back you're looking at the statistics. For example, over the past decade, the odds of a passenger dying due to an incident on a major U.S. airline have been zero (despite at least hundreds of millions of passengers being carried in that time.) Regional airlines and airlines in regions with less strict regulations and pilot experience requirements have, of course, fared quite a bit worse than that.

@reirab You might have to wait until next year to make that claim -  Delta Connection Flight 5191 shows you can't simply buy a ticket from a so-called "major" airline and expect to benefit from a decade long "perfect safety record". I'm sure you can take issue with several aspects of this crash, but ten years is a conveniently short window, so again it comes down to slicing the statistics.  Regardless, this is waaay off-topic and getting too long for a one-off silly joke I made about turbulence reminding me of roller coasters.

@AdamDavis Comair is regional airline, not a major airline. Like I said, the regionals don't have quite as pristine of a record, though it's still actually quite good.

To illustrate serious turbulence and how dangerous it can get for passengers - Etihad turbulence: More than 30 passengers injured: "... 22 people had been treated by paramedics for minor injuries at the airport" / "at least eight Indonesian passengers and a foreign flight attendant were hurt, with several suffering broken bones". This is quite rare though.

@Czechnology: you can be sure that all those people got injured because they were not buckled up. Outside of northern Europe/North America, it is still pretty common for people to unbuckle as soon as the light is out.

This is a great question. It’s something that goes through every passengers mind during turbulence and great to have more insight!

Very little to go on (non-pilots are lousy at classifying turbulence) but my best guess is that you experienced moderate turbulence.
If it was severe, your description would have been something more like this:

I couldn't focus my eyes to see. Someone who wasn't belted down was
  flung violently against the ceiling, then slammed to the floor. The
  captain was very assertive, telling the FAs to be seated immediately and
  that all passengers must strap in. The flight attendants wedged the
  service carts into the gaps between the seats and asked some
  passengers to hold them down, before sitting down and securing
  themselves in with their full 4 point harness. During the worst of it, there was no way anyone would have been able to stand, even if you were holding onto the seats or walls. Some overhead bins
  popped open strewing their
  contents about the cabin. There was coffee, cups, plastic glasses,
  books, etc. flying around EVERYWHERE. Several
  people had to be taken off the plane by stretcher.

Pilots are not "scared to land". If it's not safe, they will divert. If the approach is wrong, they will "go around" (both perfectly normal) and spacing issues are the most likely reason. Why would the captain lie?
Ask a pilot.
The reaction of passengers is not a good indicator of the level of turbulence but the bottom line is, aircraft are designed to cope with the worst turbulence, and then some. You were perfectly safe.
I have to take issue with your use of "air hostesses", I believe you mean "cabin crew" or "flight attendants".  We stopped calling them that when we realised that they are actually professionals with a vital safety role as their primary function and are of both genders.

Well it feels good to hear I was safe - I felt the rest of the passengers were overreacting but I wanted to know how to gauge the situation for my own personal benefit. As far as "air hostessess" I could not gauge the reaction of the whole crew, only the two female flight attendants nearby me so that is why I used the term. Thank you for updating me to modern terminology.

How did it compare to this?.  This is moderate.

I would say it was about comparable to the majority of the ride while the bigger dips I described would have been a bit worse than that (and it was the unexpected nature of it that made it unsettling). By the way, I mentioned the "air hostess" issue to my family and they all laughed at me and asked if I was from the 1960's. No idea where I got that from :-(

Are not flight attendants still called air hostesses in European English?

@Dronz.  I am as European and English as can possibly be.  Maybe a generation ago.  Not now.  It's pejorative.

A captain has very good reasons to lie. Passengers freaking out aboard a plane has led to crashes due to weight imbalance

In France, we still call them hôtesse and steward, as demonstrated by this profession profile on the website of the State agency for orientation after school. That's indeed pejorative and this is the way they are seen by many passengers.

@MarchHo You're using that example to demonstrate that captains have good reason to lie?  Really?  What does that incident have in common with this story?

The incident is unrelated, but it is not inconceivable that a captain would lie in order to prevent passengers from freaking out and running about the plane, causing additional difficulties in controlling the plane.

@MarchHo Sorry, but your point is both unlikely and completed unfounded - even as someone who "gets" weight and balance, it's still a hell of a decision when faced with something like that. On the contrary - where exactly do you suppose the customers are going to run to on a turbulent plane? Most of the evidence points towards the idea that actually passengers will generally just sit tight in an emergency, accepting that the pilots have just as much riding on it as they do. Pilots may not choose to give all the information out, but I doubt they'd have reason to lie outright.

@Dan From the perspective of the passenger, I am in agreement with you here on all points. I feel bad for accusing the pilot now but I reacted from fear when I wrote this message in the same way that I imagine many passengers would act in this situation. So in a way I'm happy that fear is represented in my original question - other passengers may relate to how I felt. We put our faith in the pilot but when things suddenly feel like they are going wrong, it's easy to start doubting everything. I know there's nothing for me to do but at least next time I will feel better about it!

Care to add details to your reference to severe turbulence?

@user568458 Read the first link in my answer to the scale of turbulence and the FAA definition. Of course, a Google search will yield both definitions and descriptions of encounters with severe turbulence but please do ignore tales from passengers.  The overwhelming majority of passengers are very bad at classifying turbulence.  Terry's description is very good - "I can't read the instruments because my eyes are jiggling around so much".  Now ask yourself, have you ever been unable to focus your eyes in turbulence?

@JoelM. As above.

All I can see that is close to a passengers' perspective description, both in that link and when I googled it earlier, is "...forced violently against seatbelts... Unsecured objects are tossed around. Food service and walking are impossible". To me, that matches the "big drops" described in the question - that's why I wondered what differences you'd expect to see in a passenger's  description. So "unable to focus eyes" would be one example?

@user568458 That's the problem.  OK, you can't focus your eyes.  Anyone not belted in will be thrown violently against the ceiling, then slammed to the floor.  If there are any service carts not secured, the flight attendants will wedge them into the gaps between the seats and ask 2 passengers to hold them down.  All cabin crew will be seated with the full 4 point harness secured.  The engines will be throttled back to slow down to turbulence penetration speed and the captain will be seeking an urgent level change.  Some overhead bins will pop open and bags will be thrown from within them.

The captain will make a very assertive call telling the FAs to be seated immediately and that all passengers must strap in.  There will be coffee, cups, plastic glasses, books etc rising up, then being thrown onto the floor.  Have a look at typical injuries and the state of the cabin following a severe turbulence encounter.  Very few people will ever experience it.  Many pilots have never experienced it.  Most people describe moderate turbulence as severe out of ignorance of what turbulence is and how it's classified.

Compare this with what a passenger describes as "insane".  What's actually being experienced is moderate.  I wouldn't even say at the high end of moderate.

Here's another aftermath of severe turbulence.

@Simon Sorry, but that video reminded me of all the old Star Trek episodes where the bridge shakes violently and everyone is flailing back and forth trying to keep their balance, but it's really just the camera shaking... :-)

Why do the politically correct terms always double the number of syllables used?

@ChrisK Why do people describe progress as "political correctness" which is just some vague made up term usually used to hide bias?  I see nothing "politically correct" about recognising that cabin crew are a vital part of the safety chain.

@Simon: I'm not denigrating progress, but why is it always double the syllables, why can't we come up with shorter terms for things?  But what do you call them individually when trying to get their attention? Miss?  Sir?   What's the right thing here?   I really don't get what's the big deal with not just using Steward and Stewardess.  I definitely realize they're not my personal wait staff, but let's face it - Flight Attendant is most definitely a politically correct assignation.

Anyone's input on my related question here is appreciated: https://aviation.stackexchange.com/questions/93635/do-airlines-record-the-plane-roll-yaw-pitch-angles-during-a-landing-eg-every

@mins Are you still hearing this? PNC or “personnel navigant commercial" seems more common to me, at least in a professional context, and I wouldn't take ONISEP as a reference.

@mins I know what ONISEP is and it's easy to find other examples (somewhat more convincing than a random préfecture), e.g. ministry, Air France. Just like in English, it doesn't work as well as a profession name (it feels awkward to say “I am a cabin crew”) but in my experience, PNC is in fact becoming much more common. I am pretty sure INSEE will catch up eventually (this classification is from 2003).

@Relaxed: Yes, but compare the number of results when searching  PNC (1,240) and hôtesse/steward (37,300) on tripadvisor.fr, with appropriate filters to remove unrelated uses of words. PNC counts for 3%. Not scientific indeed. I'm not saying this is how professionals talk, only medias/customers.

I agree with @Simon that, given your description, you were in moderate turbulence, and as such you were not at risk.
You mentioned that the flight attendants looked terrified, which implies that you were able to see their faces, which might mean they were up and about? If so, you were not in severe turbulence. In severe turbulence the flight attendants are going to be strapped down. My guess would be that what you saw on the the flight attendants' faces was irritation rather than terror, unless they were newbies.
As Simon mentioned, passengers are poor reporters when it comes to turbulence. I was in the back of a 727 well aft of the wing over Texas many years ago. They stopped the beverage service when a flight attendant got dumped onto a passenger's lap. I already had my carton of milk (I'm a milkaholic). I was trying to drink it between bumps without much success, and I found my efforts laughable and started laughing. The passenger next to me asked how I could laugh at a time like this when we were obviously in great danger. I said something like, ah, what danger? to which he replied that I should look at the wing tips, that they were moving up and down. Whether my explanation that they were supposed to do that and that we were in no danger really consoled him or not, I do not know.
It's an oversimplification, but the wing is designed to stall before it will overstress the aircraft, regardless of the turbulence. People have been occasionally hurt in severe turbulence, though, by flying about because they didn't fasten their seat belt, or by overhead bag compartments opening and bags flying. It's very rare though, and preventable.
Large aircraft typically have a recommended turbulence penetration speed to ensure margins for both the high-speed buffet and stall speeds. The 747-100/200 aircraft also had a TURB mode for the autopilot for use in moderate to severe turbulence, although I always found I could do a better job hand-flying than those old autopilots could insofar as handling turbulence. I would guess that current autopilots can do a better job than hand-flying.
My personal way of judging whether we were in truly severe turbulence was that if I was having trouble reading the instruments because my eyeballs were jiggling too much, that was severe turbulence. Also, if you're in it long enough, when the flight is over, the area around your hips may be a little sore for having been forced against your seat belt so hard and so often.

So, as a practical matter the greatest turbulence danger is when it is unexpected; when many passengers are not strapped in and there are many loose items about the cabin. That turbulence is likely brief, relatively light yet may cause injuries.

@radarbob Though I don't have the statistics at hand to prove that first sentence, I believe it to be true. Concerning the 2nd sentence: CAT, clear-air-turbulence is usually brief, but I wouldn't characterize turbulence that lifts unsecured items and displaces them as light.

In general my original question (and the fact that I doubted the pilot) has made me feel a little bit silly for asking now. This answer gives me perspective and makes me feel a lot more comfortable about turbulence and what to expect in a flight like this. Given the reaction on the plane and what I expect the average passenger might feel I hope my question and these great answers might put them at ease. As far as the flight attendants, they were strapped down but I could see their faces if I looked out down the aisle. I may very well have misread their expressions.

I have actually seen some seriously startled FAs in moderate turbulence before, but that was when the pilots didn't tell them to stop service and sit down before we started entering the jet stream on a flight back from Asia. They sat down very quickly after that first drop. - lol

@reirab If it was clear air turbulence, the pilots may not have known it was coming. I'd certainly be startled if I was a FA and things went from stable to service carts abruptly coming off the floor or nearly so.

It's when the wing tips stop bouncing up and down that I would start to get worried.

@Simon - is that because your assumption would be that the wing has probably stalled in order to no longer be buffeting?

@JonStory No, I was just joking.  It's a spin on the fact that since the wing tips are flexing up and down, the wing is doing it's job as designed and absorbing the flex forces.

Ah ok, I got that it was jokey but wondered if it had a basis in a nuance like the stall...thing

@radarbob: In a stall lift stops to increase with angle of attack, but plenty is left to cause loads on the wing. Zero g load is only possible in a parabola flight on the Vomit Comet, or on the ground when the wing is at rest.

Turbulence can be caused by a variety of different weather patterns.
The one you describe in TX as "an area of dense clouds and lightning so thick that there was no visibility outside the windows" is probably a thunderstorm.
Inside the cockpit, pilots have a weather radar system that tells them instantaneously where the densest area of precipitation is, such as the magenta area at the far right of the 40nm arc. Aircraft equipped with radar can easily fly around the worst parts of the storm, but that doesn't mean they miss it all together.
Your pilot probably had to fly through part of the storm to get to the destination. The go-around could have been for 'spacing,' but it could have been a normal missed approach as well.

You can tell I'm not a pilot. I read 40nm as 40 nano meters and thought radar wavelength before realizing that really meant 40 nautical miles away.

@FreeMan just multiply by 1.852e+12

hmmm... haven't had enough coffee to work that one out...

When I was in training, we had specific explanations on the effects of the different levels of turbulence.
Light turbulence:
Pilot's point of view:
Turbulence that momentarily causes slight erratic changes in altitude and/or attitude (pitch, roll and yaw).
Passenger's point of view:
Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered in walking.
Moderate turbulence
Pilot's point of view:
Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed.
Passenger's point of view:
Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.
Severe turbulence
Pilot's point of view:
Turbulence that causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control.
Passenger's point of view:
Occupants are forced violently against seat belts or shoulder straps.
Unsecured objects are tossed about. Food Service and walking are impossible.
To comment on the "going around" for another attempt. This is always ok. Perfectly normal. There can be countless reasons why this might happen.
And when it happens, the captain has decided that it is safer to go around than to continue.
The only place turbulence may be a problem is close to the ground (ie. on approach to land). If there is a sudden downdraught close to the ground, there may not be enough height to recover. This has happened in the past due to a microburst but most modern airports now have specific radar designed to detect these phenomena.

If all the flight attendants are strapped in, injured, or crying uncontrollably, and preferably all at the same time, then you probably can go off the flight attendants. Maybe. In all other situations you should not. Even if you correctly detect stress it could be for a whole more reasons than the turbulence you are experiencing - because they fly so often, they will have experienced worse, many times before.
I think this is a good informative article that might help http://www.askthepilot.com/questionanswers/turbulence/
You have a trained professional crew up front, who again will have experienced this many many times, they might not be happy about it because they want their passengers to have a smooth ride at all times, but refer to my first sentence before you even worry about the flight crew getting upset or crying also or it affecting their professional judgement in any way whatsoever. Also its far easier in these situations if you are driving than just sat in the back waiting for the next bump.
If you are on the sea you expect the odd wave and can see it coming, its difficult in the air because you can't see the wave coming. I much prefer sitting in the cockpit myself than sat in the back, in particular if the weather is rough. You would be amazed at how calm it is no matter what the weather (or anything else for that matter). The pilots will be totally relaxed. The only time turbulence will really get their attention is when on approach - the closer you are to the ground when it happens the more attention the pilot will pay to it.
Remember the wings rock when the pilot wants to adjust the direction you are going and the wind often changes more than once on approach (a lot more if its gusty), so its the pilot moving the wings in that case to keep the nose of the aircraft pointing in the right direction because he/she has to do that to change the direction you are heading when the wind blows it off slightly. The wings are used rather than the rudder because its the correct technique, if the rudder was used, now that really would be uncomfortable (and potentially dangerous). Also if the pilot doesn't do this, you will miss the airport by a mile. So what you think is turbulence on approach might be a bit of both - the turbulence itself and the pilot also telling the aircraft exactly where it needs to go. A good game to play is to try to work out which wing rocks are the pilot and which are due to turbulence. The pilot will be smoother (hopefully).
The only other thing I would like to add which I hope will help, is that if you watch a video of airliner wings being stress tested, the wingtips more or less have to touch each other before the wing breaks. Not quite perhaps touch but they move a whole lot further than you will ever see in flight.
Severe turbulence can injure people walking around because it could pin them to the roof and then drop them to the floor the next second. Thats why you have the seatbelt signs on at the first sign of turbulence. However, even if this happened and dozens of passengers were injured in this way, the aircraft would be totally undamaged. Obviously something the pilots will try their best to avoid.
I hope this answer helps. Please just relax and enjoy the ride, aircraft are not designed to fall apart in the air the first bump that comes along, they fly for thousands of hours in situations like this and sometimes pilots just can't avoid the turbulence if its one of those days where its everywhere!

Turbulence is part and parcel of flying. Rest assured that pilots are trained to handle all types of turbulence in the simulator training. Airplanes are equipped with radar to detect precipitation which usually cause turbulence.

"pilots are trained to handle all types of turbulence", but they can do mistakes when dealing with them and brake the plane: The NTSB concluded that the enormous stress on the vertical stabilizer was due to the first officer's "unnecessary and excessive" rudder inputs, and not the wake turbulence caused by the 747 (AAL 587)

authors: ابوعلي الغابي


text:
Is ETOPS satisfied solely by range circles?
Consider a scenario where an airplane is operated on an ETOPS 180 route where the ETOPS alternate is open, available and above minima, but there is terrain between the route and the airport which makes it absolutely unreachable. If, in that scenario, an engine failure or decompression alternate is available but more than 180 flying minutes away, and enough fuel is carried to reach it while satisfying the terrain clearance requirements, would that flight be legal (not: practical, safe, desirable - only legal!) according to EASA/FAA regs? In other words, is ETOPS satisfied solely by looking at range circles around available airfields, or are there additional considerations required (not: prudent, desirable - required!) by regulations?

Maybe I misunderstand, but don't ETOPS regulations apply to flying over water, while terrain between the route and the airport would imply you're not over water, but over land, thereby making ETOPS a moot point?

As far as I can see, ETOPS was originally developed for flying over water, but equally applies to flying over land, as fundamentally there is no difference in flying over water or over a vast desert - neither areas provide suitable diversion options.

No, ETOPS is not satisfied solely by range circles, but (among other things) by the "time needed to fly to that airport".
See the requirements of 14 CFR 121.633 (emphasis mine):

(a) For ETOPS up to and including 180 minutes, no person may list an airport as an ETOPS Alternate Airport in a dispatch or flight release if the time needed to fly to that airport (at the approved one-engine inoperative cruise speed under standard conditions in still air) would exceed the approved time for the airplane's most limiting ETOPS Significant System (including the airplane's most limiting fire suppression system time for those cargo and baggage compartments required by regulation to have fire-suppression systems) minus 15 minutes.

Paragraph (b), for ETOPS beyond 180 minutes, gives the same "time needed" requirement.
So, if a scenario existed as the question describes where "there is terrain between the route and the airport which makes [the ETOPS alternate] absolutely unreachable", then that flight could not reach that alternate in the required time (since the alternate is not reachable), and the flight could not be dispatched listing that alternate.

Alright, that makes sense. On the other hand, the "time needed to fly to" that airport is an academic exercise, anyway, isn't it? Or will the circles be adjusted for wind, even though the regs state no wind? What about an event after which the OEI planning speed is not feasible? Or if the speed needs to be brought back for turbulence penetration? Or, similarly to terrain but crucially different, what if enroute weather prevents direct flight to the ETOPS alternate from the route?

@CptReynolds I doubt the FAA would buy the "just an academic exercise" argument.

Well... you're not required to actually fly the diversion at exactly that speed, correct?

@CptReynolds I don't suppose so. These regs are for dispatching the flight.

The regulations are about defining a maximum - at the end of the day they're arbitrary to some extent: what's the real difference between 170 and 190 minutes on one engine? The point is that all aircraft are held to the same standard, which has been decided on as being safe (or rather: reasonably safe). If your last engine gives up at 170 miles or 190 miles, you're still probably ditching/finding somewhere nice and flat to try to land.

This isn't ETOPS specific, but is related to high terrain.  When overflying terrain that limits your ability to reach your destination or destination alternate you plan additional alternates called "drift-down" alternates.  The flight plan is then segmented into drift-down segments that determine which alternate you will land at if needed for that segment of flight.
The drift down information will be part of the dispatch release and assures that from any point on your route with an engine failure you can  meet obstacle clearance requirements while drifting down toward a suitable alternate airport.
Routes I flew between the US and Mexico often had drift-down alternates as there were areas on many of our routes that we could neither return to our departure airport or make our destination if we experienced an engine failure.  In those cases we'd reference which drift-down segment we were in and head toward the alternate for that segment.
The regulatory basis for the drift-down procedures and alternates is 14 CFR 121.191.  The synopsis is that flight with a failed engine must be able to proceed to a destination or alternate from any point along the route while maintaining specific terrain clearance requirements.  The "drift-down" alternates must meet the same requirements as normal alternates (weather, suitability) and must be present in the dispatch release.
Combining this with ETOPS, the drift-down alternates would have to be within ETOPS range in addition to the terrain drift-down requirements.
The regs:

§121.191   Airplanes: Turbine engine powered: En route limitations: One engine inoperative.
(a) No person operating a turbine engine powered airplane may take off that airplane at a weight, allowing for normal consumption of fuel and oil, that is greater than that which (under the approved, one engine inoperative, en route net flight path data in the Airplane Flight Manual for that airplane) will allow compliance with paragraph (a) (1) or (2) of this section, based on the ambient temperatures expected en route:
(1) There is a positive slope at an altitude of at least 1,000 feet above all terrain and obstructions within five statute miles on each side of the intended track, and, in addition, if that airplane was certificated after August 29, 1959 (SR 422B) there is a positive slope at 1,500 feet above the airport where the airplane is assumed to land after an engine fails.
(2) The net flight path allows the airplane to continue flight from the cruising altitude to an airport where a landing can be made under §121.197, clearing all terrain and obstructions within five statute miles of the intended track by at least 2,000 feet vertically and with a positive slope at 1,000 feet above the airport where the airplane lands after an engine fails, or, if that airplane was certificated after September 30, 1958 (SR 422A, 422B), with a positive slope at 1,500 feet above the airport where the airplane lands after an engine fails.
(b) For the purposes of paragraph (a)(2) of this section, it is assumed that—
(1) The engine fails at the most critical point en route;
(2) The airplane passes over the critical obstruction, after engine failure at a point that is no closer to the obstruction than the nearest approved radio navigation fix, unless the Administrator authorizes a different procedure based on adequate operational safeguards;
(3) An approved method is used to allow for adverse winds:
(4) Fuel jettisoning will be allowed if the certificate holder shows that the crew is properly instructed, that the training program is adequate, and that all other precautions are taken to insure a safe procedure;
(5) The alternate airport is specified in the dispatch or flight release and meets the prescribed weather minimums; and
(6) The consumption of fuel and oil after engine failure is the same as the consumption that is allowed for in the approved net flight path data in the Airplane Flight Manual.

With your opening clause, are you stating that your answer is not ETOPS specific, or are you referring to the question?

@JonathanWalters both in a way.  He asks about ETOPS over terrain, but the driftdown alternates will be necessary whether or not you are flying under ETOPS rules -- they are necessary if significant terrain is present that you can't clear with a failed engine.


text:
How do police/ambulance helicopters communicate with ATC?
I was watching some police programme on TV the other day, with an air chase that had the police helicopter crew on their toes; having to perform a lot of sudden maneuvers.
How do police, or HEMS (medical), helicopters communicate with ATC? I presume they get priority, but do ATC clear other traffic out of the way? Is there a comms person/navigator on-board? Do they simply "see and avoid"?

They would operate below controlled airspace so no need to talk to ATC unless they get close to an airport.

@ratchetfreak The majority of the UK is very close to an airport.

The exact procedures will vary locally but typically ATC will give priority to police and ambulance helicopters that operate emergency services.
If these helicopters are operating in controlled airspace, communication with ATC will be done by normal VHF radio. There is no need for a dedicated communications person, the pilot will be in contact with ATC.
Sudden manoeuvring of these flights is rarely a problem for ATC since these aircraft are operating under Visual Flight Rules.
When chasing a car, the pilot remains responsible for keeping clear of obstacles and other traffic, therefore this requires a second person to keep a look out (either on the car or the surroundings)

are there two radio buttons, one for aviation freqs, one for emergency services?

@rbp I assume there is a two button setup, I am not sure. I'll try and find out but this might take a while as I am not in regular contact with these crew.

@rbp.  Generally not.  The pilots do not communicate with the emergency services.  There is an observer on board, sometimes occupying the left hand seat, sometimes in the rear, who is communicating with the ground forces and operating the video recorders, IR sensors and so on. http://content.met.police.uk/Site/adayinthelifeofasu

I just found an AIC (POLICY FOR THE APPLICATION OF CALLSIGNS TO HELICOPTER EMERGENCY MEDICAL SERVICE FLIGHTS) of relevance.
HEMS flights are prioritised by ATC:

The Flight Category is a priority status allocated to a flight by the Civil Aviation Authority (CAA) designed for use with tactical
  handling by ATC. There are 7 categories of flight as defined in MATS Pt 1 page 1-31. They are: Category A-E; Normal flights and
  Category Z. The flight categories relevant to HEMS operations are:

Flight Category A - the highest category of flight, applies to all HEMS flights, as defined at paragraph 2.1;
Flight Category E - is authorised for use by an aircraft positioning for the purpose of conducting HEMS duties, eg
  returning to base after delivering a casualty to hospital. It is afforded priority over normal flights;
Flight Category Z - is authorised for flights involving the training of crews for HEMS operations.

perhaps they should have blues and twos on them!


text:
After an emergency landing, what procedures should the pilot follow?
In case of an emergency landing (non-fatal) on a road/highway or in a large field, what happens next?

Should police be contacted?
Should aircraft owner and/or insurance be contacted?
In case landing is on a road or highway, what should the pilot do?
In case of landing on someone's personal property, what should one do?

Rules and regulations are different in different countries/states/jurisdictions, are you asking about a specific one, or are you asking generally?

Well it's a general knowledge question but I'm more interested in USA.

(I know this is an old question, but...) kiss the ground.

In this case, "Wake up!"  https://www.faa.gov/pilots/safety/pilotsafetybrochures/media/CObroforweb.pdf

Put on clean underwear?

There seem to be two parts to your question. First, what practical steps can I take on the ground immediately after landing? Second, which authorities must be notified?
For the first point, there are just too many scenarios to have a single answer but basically you should immediately check for injuries or damage to property and activate the emergency services if required. After that, secure the aircraft to prevent any further damage, call your insurance company and inform the aircraft owner and local property owner (as applicable). I'd also suggest giving your family a call rather than have them hear on TV news that you were "in a crash landing".
There are more dramatic possibilities, of course: if you ditched or landed in a remote area or in extreme weather then actual survival could be your highest priority. But that's a topic for another site.
For the second point, at least in the US it's possible that you don't have to notify anyone. If no one is hurt, nothing is damaged, and none of the incidents requiring immediate notification occurred (see 49 CFR 830.5 and the AIM 7-6-2) then there's no legal requirement to inform anyone. The authorities in this case would be the NTSB, not the FAA, by the way. There's a more detailed discussion of these requirements here.
An additional concern that I know nothing about is liability and other legal issues. Getting legal advice - e.g. from your insurance company or AOPA - would probably be a good idea whatever the circumstances.

Excellent answer, also nice split general/US

Outlandings are a regular occurrence when gliding cross-country. During competitions with tens of participants, it is not uncommon for a few gliders to land out each day. For this reason, in the UK at least, glider pilots are taught how to land out, and think about this eventuality on every flight. Many will have experienced a landout with an instructor before going solo, and practice approaches to fields are a requirement of gaining the cross-country endorsement (part of the UK syllabus).
In practice, there isn't a lot to do after your beautifully-executed landing. In common law countries (including the US and the UK) on private land you are trespassing, and the landowner can ask you to leave. Of course this is already your intention, but it's complicated by the expensive machine you have just deposited there. Any damage you cause (or caused, on your way in) is your liability. Your aircraft is still yours, and the landowner cannot prevent you from removing it. If he tries, he is responsible for keeping it safe and will be liable for any damage. He can charge a reasonable fee for his time and the unauthorised use of his land, but often will not. Some landowners object to professional balloon companies charging large sums for taking parties of champagne drinkers into their fields and want a cut of the profits. In this case, remind him that you are a private individual and that you landed there without intending to in an emergency.
So your plan should basically be:

Make the aircraft safe.
Inform whoever you were most recently in contact with that you have landed safely (ATC, or your fellow travellers on common frequencies).
Find the landowner, explain that you didn't mean to land there, that it was an emergency and that you intend to retrieve your property as soon as possible. Obtain permission for vehicles to enter his land.
Contact your friends to bring the trailer, derig the aircraft and drive home.
Buy dinner for your friends.

If you have approached over built-up areas or roads, someone will probably have seen you and is likely to call the emergency services on your behalf. In the UK, NATS (the National Air Traffic Service) coordinates with the police in cases of aircraft outlandings. You should call their distress and diversion (D&D) cell to inform them of the situation, even if there is no damage or injury.
If the landowner tries to impound your aircraft or becomes aggressive, call the police. There have been cases where landowners have caused damage to aircraft and recourse was only possible because of police attendance.
If you have caused any damage, try to come to an agreement on the cost. Often, young crops are not damaged by landing aircraft, and if you have landed in a long crop you are probably not capable of discussing compensation. Ploughed fields and grass fields are more likely to have damaged your aircraft than the other way round.
Outlandings are not the end of the world. If it worries you, why not hop along to your local gliding club and take a flight. Most instructors will be more than happy to show you round the local fields!

I guess one difference here is that gliders are designed to be derigged and trailered out. A powered aircraft with a dead engine may be somewhat more complicated to remove.

You're right, but by coincidence I've been learning to fly motor gliders and mine happened to have an engine failure and landed in a field! Some syndicate members arrived by road with the required bits and the aircraft was flown out with the landowner's permission. Our insurance does cover retrieval if that weren't possible, but practically it probably involves quite a large trailer capable of safely carrying the bits once it's been derigged.

You are talking explicitly about an emergency landing, not about an outlanding (yes, this word really seems to exist...).
Here's the answer that is valid in Europe:
In case of emergency, the federal office of aviation has to be informed. They will then check the conditions. If you start before their OK, it will be a criminal act.
If it was a security landing, you will need to ask the owner of the land for permission and you'll need somebody who's watching while you take off.
And no, you don't have to inform the police, but usually you will.

I've never heard "outlanding", I think "off field landing" or "off airport landing" are more common names

Nice answer. I wish I could mark this too as answer.

"Outlanding" is the term used in gliding.   It describes a relatively routine, if not completely intentional event, which happens when a glider pilot can no longer stay aloft.   "Off field" or "off airport" are phrases that sound like a power pilot talking about a rather different occurrence: either an emergency, or a very unusual call to land somewhere where there isn't an airfield.

in US soaring, we call it "landing out"


text:
What are the criteria that govern the choice between titanium, composites, and more common materials, like aluminum?
In the newest large commercial jets, composite materials seem to be all the rage. The first thing I wanted to know is why composites were chosen instead of titanium? If I'm not mistaken, titanium has even more specific tensile strength.
Let's also be clear about what exactly composites are. "Composite" just means a mix of two or more materials, and in aerospace, it is predominantly autoclave-cured carbon-epoxy.
Ultimately, it would be good to understand the criteria that govern any choice among titanium, composites, and aluminum. Specifically, which areas of the plane are candidates for an advanced material? and why? What factors dictate the decision?

Aerospace composites include fiberglass, resins, epoxies, and phenolics, in addition to carbon.

The choice between all of those materials is extremely broad... is there a certain application you are interested in?

The cost of a titanium airliner would be ridiculous...

@RonBeyer Why is that exactly? Titanium used to be rare but today we've invented cheaper refining processes, and IIRC, titanium oxide is the 9th most abundant compound in the Earth's crust.

so far i have 7 views, 4 close votes, and no explanation(s) why.

@fooot a certain application? Well yes, the application of aircraft of course. Just off the top of my head, I think common places for titanium are wing leading edges, nose tip, and inside the engine. Those are probably as good a place to start as any.

Titanium is roughly 4.20 USD/kg, whereas aluminum is roughly 1.20 USD/kg. Composites are even cheaper, not to mention lighter weight. I wouldn't base the issue on tensile strength alone, cost, tooling, repairability, availability, and weight are big factors. The reason I gave a close vote as too broad is that really this question would require a course on material sciences along with aircraft design, and can't be succinctly answered in a few paragraphs.

@RonBeyer I don't think it is too broad. How do we know it can't be answered succintly unless we give someone the chance of trying? Closing the question would close that chance.

Keeping in mind that an alloy is a composite of two or more different metals, aircraft are built almost exclusively of composites.

What are the criteria that govern the choice between titanium, composites, and more common materials, like aluminum?

Three main criteria: cost, strength-to-weight ratio, and fatigue resistance.

Cost. Of the three, aluminium used to be the clear winner, with composites having made large advances due to improved manufacturing processes. Titanium is the most expensive and is difficult to machine.
Strength-to-weight.

Buckling. Titanium alloys have higher strength-to-weight than aluminium (this answer, more on temperature below), but aluminium is lighter and that gives it an advantage in structures loaded with compression stresses: buckling resistance is a function of cross sectional dimension as well. For the wing upper surface aluminium would be lighter than titanium, despite the lower specific strength. Composites have the highest specific strength of all.
Temperature. The graph in the linked answer also shows the influence of temperature on the specific strength of materials: aluminium is first to drop, and at higher temps titanium is the best next choice, as from the comment by @PeterKämpf.

Fatigue resistance. Titanium alloys, like steel allows, have a fatigue endurance limit. If stresses remain below this limit, the construction can endure an endless number of cycles. Aluminium does not have an endurance limit, and will eventually fail even from small stress amplitude cycles: aluminium constructions require careful monitoring and maintenance for prevention of fatigue failure.

Composites do not have an endurance limit, but fibre orientation and material choice can improve fatigue life.

Image source

Good answer. What is missing is temperature: The loss of strength of aluminium with temperature prohibits its use areas like near the engine exhaust, and titanium is  often the next best choice. Titanium is not only expensive, but also expensive to work with. It wears down tools much more quickly and needs extra attention when being welded so no heated surface is exposed to oxygen.

"The application of aircraft" is extremely vague. Modern aircraft use all of these materials in many different places, for many different reasons. Each component will have different tradeoffs that depend on many factors. The following is an extremely general overview.
Aluminum is a popular material in aircraft because it is relatively cheap and light, and has alloys with good material properties. It is fairly easy to work with but must be protected from corrosion. Light weight and low cost mean that it is used in large areas like fuselage and wing skin, and for a lot of the underlying structure.
Titanium is useful for its ability to withstand higher temperatures, while being stronger than aluminum but also heavier. However, it is much more expensive than aluminum.
Composites are a large family of materials, with many different types and combinations possible. Composites can be strong and light, but don't withstand high temperatures as well. Although composites don't corrode like some metals, some situations such as carbon fiber contacting aluminum need to be avoided. Exposure to UV light or moisture can also be an issue. Manufacturing composites can become very expensive depending on the materials used. Since composites are typically manufactured from multiple layers sandwiched together, they lend themselves more easily to applications with large and thin sections. Larger and more complex parts are more difficult to make from composites. Another important factor with aircraft is electrical conductivity. While metal parts will naturally conduct electric charge between each other, composites will need special treatment to ensure conductivity for protection from lightning and static charges.
There are also many other considerations. Besides yield strength, many materials on aircraft need to have good fatigue properties to withstand cyclic loading over time. Material properties at high and/or low temperatures may also be important. While metals tend to bend and dissipate energy before breaking, composites tend to suddenly snap. Metals are also easier to inspect and repair, while composites can be much more complicated. While a metal can be categorized fairly well by its type and dimensions, composites are more complex with their multiple plies. This makes definition and analysis more complicated.

The first question is decently answerable.
Composites have the nice property that they are not homogeneous at a mesoscopic scale. It's almost unavoidable that small cracks form in materials under stress. This happens in aluminium, titanium and composites alike. This is not dangerous if there's something to stop them from growing. Within aerospace composites, the local material boundaries stop crack growth. Practically speaking, for a laminar composite this means that one layer may develop a crack. The two adjacent layers will stay glued to both sides of the crack, and keep the panel together.
Composites are also easier to engineer for some specific purposes, such as having higher tensile strengths in critical directions. This isn't impossible for titanium, but it is hugely expensive. You have to grow and cut a single crystal of titanium. In comparison, with a carbon composite it's just a matter of orienting the fiber layers.

Can you explain exactly what "grow and cut a single crystal of titanium" means? Can't titanium just be welded?

@DrZ214: You can do so, but the resulting structure will be a hodgepodge of lots of little crystals oriented in all different directions.  Many to most, if not all, crystalline materials are strongest against forces applied parallel to one of their crystal axes.  If your structural panel is machined from a single huge crystal, or from multiple crystals all with the same orientation, then the panel will be strongest in one particular direction, and you can orient the panel such that the direction in which the panel experiences the most stress also happens to be the direction in which (1/3)

the panel is strongest.  If, on the other hoof, the panel is made up of lots of little crystals with random orientations, the crystals' strongest axes will point in all different directions, and the panel will have the same strength in every direction - which is less than the strength of the monocrystalline (or polycrystalline with aligned crystals) panel in its strongest direction.  A monocrystalline panel, or a polycrystalline panel with all the crystals pointing the same way, can be made so that it is strongest in the direction in which it needs to be the strongest, (2/4)

rather than wasting strength in directions in which it isn't needed.  In contrast, a polycrystalline panel with isotropically-oriented crystals will be the same strength in every direction - which is stronger in some directions than it needs to be, yet not as strong as it could be in the directions that it does need to be strongest in.  Finally, when we compare a monocrystalline panel to a polycrystalline panel with identically-aligned crystals, the monocrystalline panel is stronger (and thus superior) to the polycrystalline panel, as it's made from a single strong crystal, while (3/4)

the polycrystalline panel is made from a bunch of separate crystals stuck together at the edges, creating weak points at the boundaries between the crystals.  (4/4)